1. Field of the Invention
The present invention relates generally to stepper motor drive circuits and, more particularly, to a circuit and a method for driving a stepper motor in an oscillating mode that is useful as the scan element in a bar-code laser scanner.
2. Prior Art
Bar-code laser scanners have employed stepper motors as scanning elements. A scanning mirror attaches to the shaft of the stepper motor and, as the stepper motor oscillates about the axis of its shaft, the scanning mirror intercepts an incident laser beam and scans it back-and-forth across a bar code.
Such a scheme requires some sort of drive circuit to cause the stepper motor to oscillate or reciprocate about its axis. Known drive circuits have suffered in that they scanned the laser beam spot at different speeds at the extreme ends of the scan than at the center of the scan, that scanning speed varied from one extreme of the scan to the other, and that scan speed varied from the forward direction to the reverse direction. These speed variations may be referred to as variations in the scan-speed profile.
Bar-code laser scanners receive the laser light reflected from the bar code, convert the reflected laser light into an analog signal, and convert the analog signal into a usable digital value indicative of the bar code. This interpretation of the returned signal requires a decode algorithm. The variations in the scan-speed profile may require more scans to yield a proper interpretation by the decode algorithm.
It is therefore desirable to have a stepper motor driven by a circuit that creates a substantially more constant higher net torque on the permanent magnet rotor of the stepper motor, which yields a more constant, symmetric scan-speed profile in both forward and reverse directions. It is also desirable to have a stepper motor driven in a constant scan-speed profile at all points in the scan.
It is often desirable to use a bar-code laser scanner in a portable, battery-operated scanning system. Such a scanner should therefore minimize power-drain on the battery and known systems have drawn an undesirably large amount of power from their power sources. It is therefore also desirable to reduce the power drain of the stepper motor.